For a fast exchange of digital signals between frequently changing remote stations, for example between telecommunications terminals, between computers serving for data processing or even between exchange-oriented processors, bus systems are frequently employed in which the respectively connected stations have random access to the bus system via transmission and reception equipment. The digital signals thereby emitted from a station are transmitted on the bus toward both sides and, based on decoding of preceding addresses, are only accepted by the station or stations identified by the address or addresses. What is referred to as CSMA/CD (carrier sense multiple access with collision detection) has gained particular significance for the control of access to the bus system; before a station begins to send, it listens on the bus to see whether a transmission event is already occurring there. When there is a transmission event, the station initially waits for the end thereof; the station then begins to send, whereby it initially continues to listen until it is certain, after a defined time span (round trip delay time), that all other stations have received its sending. When another station has begun to send roughly simultaneously in the same way, then the two stations will identify a collision of their digital signals with those of the respective other station, whereupon they will respectively abort their transmission operation in order to begin therewith a new transmission after a random time span.
A simple method with which a multiple transmitting can be identified is as follows. The active station compares the signal it is now to output to the signal statuses prevailing at its reception equipment connected to the bus system, whereby a signal status difference then indicates a collision. The recognition of such a signal status difference assumes that one signal status can always physically prevail over the other signal statuses (given binary signals) on the bus system. Regardless of the number of transmitters which are active roughly simultaneously, this signal priority must thereby also be guaranteed given great line lengths of more than 100m for every station.
A bus system (disclosed by EP-A1-017155) has two signal conductors having transmission equipment respectively connected thereto via two difference outputs and having two signal statuses one of which effects a dominance signal status in the bus system and for the other of which the appertaining transmission equipment leaves the two signal conductors at least approximately unloaded. The two difference outputs at the transmission equipment respectively formed with the bus transmitters has two difference outputs connected to the two signal conductors via two mutually opposite diodes situated in their inhibited condition in the other signal status.
This prerequisite is not met by currently commercially available standard transmitters having two difference outputs for a bus system having two signal conductors. Given such bus transmitters which are usually switched into a third, quiescent condition (tri state) in times of inactivity, i.e., outside of the bit time spans of transmitted bits, opposed signal statuses which can lead to signal falsifications or to transmitter damage due to overload can be avoided in this quiescent condition given clock-controlled or access-controlled bus systems. Given a random access bus system, the access control is based on opposing signal statuses occurring, however, such opposing signal statuses may not be recognized at all under certain conditions given the utilization of ordinary standard bus transmitters for great line lengths between stations lying far apart. The opposing signal statuses may be overlooked due to the voltage drop along the bus.
This bus system, which assumes a galvanic coupling of the transmission-reception equipment to the two signal conductors, guarantees a uniform signal status in the bus system and, thus, increased reliability in the recognition of a collision of transmitters given simultaneous access of the bus, guaranteeing this independently of the distance between and the of the number of transmitters active roughly simultaneously. Also, avoided is an overload of the sender outputs given opposite signal statuses.
Given bus systems having greater line lengths as required in local area networks (LAN), the line atenuation and the power used by subscriber stations along the bus lead to a considerable reduction of the signal amplitudes. This respectively limits the maximum allowable line length for a specific data rate and number of subscribers.
Fundamentally, the maximum allowable line length can be increased by utilizing a bus cable having an enlarged conductor cross section and, thus, having reduced atenuation. In a corresponding expansion of a bus system already installed, however, this requires a replacement of the previous bus cable by a corresponding, new bus cable, or, for a new installation of a bus system, requires a larger bus cable from the very offset, even when the initial extent of the bus system would not even make such a large cable necessary.
The present invention discloses a novel bus system having increased maximum allowable distance between subscriber stations.